This invention relates to nuclear fuel pins.
In a fast breeder nuclear reactor the core which contains the fissile nuclear fuel material is surrounded by a breeder blanket of fertile material which may include axial breeder blankets above and below the core. In one type of fuel pin suitable for use in a fast breeder nuclear reactor the axial breeder blankets are provided by fertile material above and below the fissile nuclear fuel material in the pin. Conveniently, the fissile nuclear fuel is in the form of microspheres packed inside the fuel pin by vibratory lcompaction. To ensure sufficiently high packing density within the pins microspheres of different sizes may be used. It is known to use microspheres of two or three sizes to ensure the required packing density of the fissile nuclear fuel material. In the case of a fuel pin having microspheres of two sizes the larger microspheres are placed in the fuel pin first and the fuel pin is vibrated to compact the microspheres before microspheres which are small enough to pass between the larger microspheres are added and the fuel pin is vibrated until all the fissile material is compacted.
After a nuclear fuel pin has been irradiated in the core of a fast breeder nuclear reactor it is reprocessed to separate the reusable fissile material from the products of nuclear fission. The breeder blanket and fissile nuclear fuel material are preferably reprocessed separately and so it is advantageous if the fissile and fertile regions remain separate within the pin. It is an object of the present invention to seek to prevent the intermixing of the material in the fissile and fertile regions within a nuclear fuel pin.